this is a person who is at a sporting event and is in the crowd and yells back to the cheerleaders when they yell to the crowd to spell the teams name.see eg. The reference to this in the song by Gwen Steffani is that she believes cheerleaders think that they are better than her and were talking about her behind her back and she doesn't want to be thought of as lower than they are and that is why she ends up becoming a cheerleader or dreaming of becoming a cheerleader. Its pretty stupid but I think that is what it is about.
eg. the cheerleaders yell give me a T and the crowd yells back T